This invention concerns periclase refractory grain, particularly high purity synthetic periclase.
It has been known for many years to produce periclase refractory grain by firing at high temperatures magnesia-yielding materials such as naturally occurring magnesium carbonates ("magnesite") or synthetic material such as magnesium hydroxide produced by reacting sea water or other brine with a calcium source such as calcined dolomite.
In past years, when such starting materials were of relatively low purity, e.g., about 90% MgO, and refractory periclase grain with from 10 to 15% porosity was acceptable, it was customary to take, for example, damp magnesium hydroxide filter cake formed from magnesium hydroxide sludge and put it directly into a rotary kiln where it was fired to temperatures of, for example, 1600.degree. C. It is also known to produce periclase grain of somewhat higher purity, e.g., from 90 to 95% MgO, by adding a sintering aid such as silica to the hydroxide filter cake prior to firing.
However, when it was desired to produce a periclase grain of higher purity, over 95% MgO, and particularly when it was demanded that the porosity of the resulting grain be less than 10%, it was found that the so-called "single burn" process was inadequate, and resort was had to the so-called "double burn" process wherein magnesium hydroxide was first calcined, for example at a temperature of about 1000.degree. C., to form lightburned MgO, which was then compacted and the compacts fired, for example in a shaft kiln, to a temperature of 1800.degree. C. or even higher.
Since the double burn process obviously requires more steps, and is more expensive from both the equipment and operating point of view, the industry has been looking for a method of making high purity periclase of low porosity by a single burn process. U.S. Pat. No. 2,537,014 discloses a method of making periclase grain wherein silica is added to magnesium hydroxide and the admixture fired to form a periclase having from 2 to 6% silica. Similarly, British Pat. No. 1,465,880 discloses a method of producing periclase grain by adding silica to a magnesium hydroxide slurry which is then dried, compacted, and fired to produce a periclase having about 92% MgO, about 4.5% SiO.sub.2, and a porosity of over volume 8%. It is also known, as in U.S. Pat. No. 2,487,290, to add a chromium compound (to yield Cr.sub.2 O.sub.3 upon firing) to magnesium hydroxide sludge, which is then dried, briquetted, and fired.
It has also been found that a dense, high purity periclase can be formed when the lime and silica contents of the periclase are so adjusted that there are at least about 2 parts CaO for each part SiO.sub.2 in the grain; i.e., when the predominant secondary phase in the grain is dicalcium silicate. Thus British Pat. No. 1,413,871 discloses a method of making periclase grain wherein magnesium hydroxide filter cake is dried to contain less than 10% water, sufficient finely pulverized silica gel is added and mixed therewith so that the molar ratio of calcia to silica in the sintered products is about 2, the dried admixture is then pressed, and the compacts fired at 1800.degree. C. for 1 hour.
However, for many applications it is desirable that the predominant secondary phase in the periclase grain not be dicalcium silicate, but rather forsterite (2 MgO.SiO.sub.2), with more or less monticellite (CaO.MgO.SiO.sub.2). Thus, the art is looking for a method of making a periclase grain of low porosity with over 95% MgO, less than 2% SiO.sub.2, and a lime:silica ratio of less than 1.0, by a single burn process. It is to the solution of this problem that the present invention is directed.